I suppose once you get into 32bit protected mode, and start playing around
with simple interrupts, you decide you want more and more! I mean you want
to harness all the cool little things that your computer can harness...from
probing the keyboard, to getting timing information from the 8549 timing chip.
It takes us the the heart and workings of the IBM PC, and teaches us the real
meaning of how a computer works and of course how our operating system does its
tasks.

Well what we have here, is simply, setting up the PIC (Programmable Interrupt
Controller) so that its set and ready to go. Then we disable all IRQ's and
enable only IRQ0 for the timer (8253/8254) chip, so that it calls an interrupt
each time it has counted down to zero. Each time it counts down to zero it
calls our interrupt, which simply increments our counter...and as its set to
100hz, every time the counter reaches 100, we modify the character on the
screen, so we get an alternating character at 1 second per change.

Note that when you run the simulation on BOCHs, a pc 80x86 simulator you'll
find that the timming isn't real time and the frequency you get from your timing
chip isn't the same as what you would get if you ran the code on a real pc.